Flash testing of photovoltaic modules with integrated electronics

ABSTRACT

A method for testing a photovoltaic (PV) module having an integrated power converter includes: obtaining a reference output signature of a PV module design in response to a flash pattern; applying the flash pattern to a PV module under test; acquiring an observed output signature of the PV module under test in response to the flash pattern; and comparing the observed output signature of the PV module under test to the reference output signature. Second reference output and observed output signatures may be obtained in response to a second flash pattern. The output signatures may be combined using various techniques. One or more parameters of the integrated power converter may be preset to one or more predetermined states prior to applying a flash pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/300,362 entitled “Flash Testing of Photovoltaic Modules withintegrated Electronics” filed on Nov. 18, 2011, which claims priorityfrom U.S. Provisional Patent Application Ser. No. 61/449,695 entitledFlash Testing of Photovoltaic Modules with Integrated Electronics filedMar. 6, 2011.

BACKGROUND

Photovoltaic (PV) panels are fabricated with strings of PV cellsconnected in series to convert solar energy to electric power. In somecases the cells may be arranged as a combination of both parallel andseries connections. During the manufacturing process, a variety of testsare used to determine adherence to stringent target specificationtolerances on all mechanical as well as electrical aspects of the PVpanels to ensure their long-term safety and reliability. These testsinclude a combination of visual, mechanical, optical and electricaltechniques. For high-quality and high-speed manufacturing, many of thesetechniques are automated using computer vision, robotics and electronicinstrumentation.

The electrical performance of a photovoltaic module depends on multiplefactors. These factors include temperature, solar irradiance,angle-of-incidence, type of PV-cells, air mass, etc. FIG. 1 illustratesthe current-voltage characteristic (I-V curve) of a typical PV panelunder certain operating conditions. When the output terminals of thepanel are shorted together, the output voltage (V) is zero, and theoutput current (I) is I_(SC), which is the short-circuit currentgenerated by the panel. As the output voltage increases, the I-V curveremains at a fairly constant level of current until it reaches a knee atwhich point it descends rapidly toward zero current at Voc, which is theopen-circuit output voltage of the panel.

PV panels are rated under standard test conditions (STC) of solarirradiance of 1,000 W/m² with zero angle of incidence, solar spectrum of1.5 air mass and 25° C. cell temperature. PV panels have traditionallybeen tested by exposing the panel to simulated sunlight under thestandard test conditions and collecting enough data to construct an I-Vcurve. From this data, key specifications may be determined includemaximum rated power, open circuit voltage, short circuit current,maximum power voltage, maximum power current, and temperaturecoefficients. However, standard techniques for exposing a PV panel andto 1,000 W/m² artificial illumination equivalent to sunlight may beprohibitively expensive, time consuming and in many cases impractical.For example, continuously exposing a PV panel to illumination at 1,000W/m² may cause heating of the PV cells, thereby distorting the I-Vcharacteristics to be measured for the determination of the panelperformance at STC.

To eliminate the problems caused by continuous light sources, “flash”testing techniques have been developed. During flash testing of a PVmodule, a flash of light, typically 1 to 50 ms long, from a Xenonfilled. (or equivalent) arc lamp is used, The spectral properties of thearc lamp are controlled to match the spectrum of the sunlight to theextent required. Alternate flash generation technologies can involve avariety of light sources including light-emitting diodes (LEDs). Theoutput from the PV panel in response to the flash is collected by a dataacquisition system and processed using a computer to determine thecharacteristic of the PV panel under test. The results are compared tothe target specifications with appropriate tolerances to determine ifthe PV panel performs within the required specifications. Hash testingof PV panels is possible clue to the rapid-response of the photovoltaiccells, and limited charge accumulation and storage requirements beforethe IV characterization tests can be adequately performed.

PV panels have traditionally been manufactured as independent componentsthat require external power conversion apparatus to optimize theoperating point of the panel and/or to convert the DC power generated bythe PV panel to AC power for connection to a local utility grid. PVpanels are now being fabricated as modules with integral powerconverters. On a typical PV module with an integral power converter,only the output terminals of the power converter are accessible fortesting. The output terminals of the PV cells are sealed to protectagainst environmental degradation.

Conventional flash testing cannot be used on. PV modules with fullyintegrated power converters such as power optimizers, AC microinvertersand/or diagnostic and safety related communication capabilities forseveral reasons: (1) large inherent energy storage devices in poweroptimizers, AC microinverters and communication circuits; (2) largestartup wattage requirements for allowing reliable startup of the moduleintegrated electronics; (3) algorithmic latencies of maximum powerpoint-tracking and digital control; and/or (4) connect and disconnectrequirements as regulated by the standards and utilities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the current-voltage characteristic (I-V curve) of atypical PV panel.

FIG. 2 illustrates an embodiment of a system for testing a PV modulehaving an integral power converter according to some inventiveprinciples of this patent disclosure.

FIG. 3 illustrates a flash pattern having multiple pulses and anextended output signature according to some inventive principles of thispatent disclosure.

FIGS. 4-7 illustrate some example flash patterns and correspondingoutput signatures for PV modules having integral power convertersaccording to some inventive principles of this patent disclosure.

FIG. 8 illustrates another embodiment of a system for testing a PVmodule having an integral power converter according to some inventiveprinciples of this patent disclosure.

FIG. 9 illustrates another embodiment of a system for testing a PVmodule having an integral power converter according to some inventiveprinciples of this patent disclosure.

FIG. 10 illustrates an example embodiment of a test method in which toflash patterns are applied to a module design and/or module under testaccording to some inventive principles of this patent disclosure.

FIG. 11 illustrates an embodiment in which extended output signaturesare combined in the frequency domain according to some inventiveprinciples of this patent disclosure.

FIG. 12 illustrates an embodiment in which flash patterns are combinedthrough addition in the time domain according to some inventiveprinciples of this patent disclosure.

FIG. 13 illustrates an embodiment in which flash patterns are combinedin the time domain according to some inventive principles of this patentdisclosure.

FIGS. 14A through 14C illustrate another method and apparatus fortesting a PV module having an integrated power converter according tosome inventive principles of this patent disclosure.

FIG. 15 illustrates how different preset patterns may be applied to theparameters of an integrated power converter according to some inventiveprinciples of this patent disclosure.

FIG. 16 illustrates an example embodiment of a PV module having anintegrated power converter according to some inventive principles ofthis patent disclosure.

DETAILED DESCRIPTION

FIG. 2 illustrates an embodiment of a system for testing a PV modulehaving an integral power converter according to some inventiveprinciples of this patent disclosure. A PV module design 10 includes anarrangement 12 of one or more PV cells and an integrated power converter14 to process power from the arrangement of PV cells. A reference outputsignature 16 of the module design is obtained in response to a flashpattern 18.

The module design 10 may be embodied as a physical PV module, i.e., aknown good embodiment of a design, in which case the reference outputsignature 16 may be obtained by observing the output of the module inresponse to a physical flash pattern 18 applied to the arrangement of PVcells on the module. Alternatively, the module design 10 may be a dataconstruct in which case the reference output signature 16 may beobtained by simulating the output of the module in response to the flashpattern 18.

The signatures thus obtained from real measurement of a known-goodmodule, or from a simulated data construct may only need to be obtainedonce. In practice, however, system calibration may need to be performedusing alternate reference cells, or using the known-good modules muchmore often due to parametric variation of the system, e.g., tocompensate for deterioration of luminescence of the arc-lamp orarc-flash sources with usage.

A physical module under test 20 includes an arrangement of PV cells 22and integrated power converter 24 of the same design as the PV moduledesign 10. The module 20 is tested by applying the same flash pattern 18to the arrangement of PV cells 22 and acquiring an observed outputsignature 26 of the PV module under test in response to the flashpattern 18. The observed output signature 26 of the PV module under testis compared to the reference output signature 16 through suitablecomparison logic 28 to determine if the observed output signature 26 ofthe module under test 20 is close enough to the reference outputsignature 16 to indicate whether the module under test falls withinacceptable tolerances of the target specifications. The comparison logicmay be implemented in hardware, software, firmware, etc., or anysuitable combination thereof, and may include mathematical andstatistical techniques to validate the module under test versus thereference signatures.

The flash pattern 18 may consists of a single light pulse.Alternatively, as shown in FIG. 3, the flash pattern 18 may includemultiple light pulses PULSE 1, PULSE 2 . . . PULSE N, in which case thereference output signature 16 and the observed output signature 26 maybe extended output signatures which include multiple signatures SIG 1,SIG 2 . . . SIG N. The multiple light pulses may have differentintensities and/or different pulse widths. They may be periodic in thesense that they begin or end at fixed time intervals, or the lightpulses may occur at irregular intervals.

The flash pattern may be generated with a flash lamp wherein each lightpulse is generated by a single optical impulse from the lamp.Alternatively, the flash pattern may be generated by using a shutterarrangement with a continuously operating lamp, such as a Xenon shortart lamp.

FIGS. 4-7 illustrate some example flash patterns and correspondingoutput signatures for PV modules having integral power convertersaccording to some inventive principles of this patent disclosure.

FIG. 4 illustrates an example flash pattern having a single pulse and acorresponding output signature. This output signature, as well as thoseshown in FIGS. 5-7, are shown generically but may include implicitcharacterizations of charge times for input capacitors, start-upsequences for microcontrollers, rise times for inductors, on-stateresistances for switching transistors, and other parameters as describedin more detail below.

FIG. 5 illustrates an example flash pattern and corresponding outputsignature of light pulses having uniform intensity and width andoccurring at uniform, i.e., periodic, intervals.

FIG. 6 illustrates an example flash pattern and corresponding outputsignature of light pulses having different intensities, but uniformwidth and intervals.

FIG. 7 illustrates an example flash pattern and corresponding outputsignature of light pulses having uniform intensity, but varying widthsand spacing.

The patterns illustrated in FIGS. 4-7 are just a few examples forpurposes of illustration. Countless other patterns and combinations ofpatterns may be implemented according to the inventive principles ofthis patent disclosure. For example, a flash pattern may also includemultiple pulses having varying intensity and non-uniform durationsbetween pulses.

Whereas prior art techniques for flash testing of PV panels obtainexplicit I-V characteristics of the arrangement of PV cells on thepanel, the output signatures acquired during testing of a PV modulehaving an integral power converter according to the inventive principlesof this patent disclosure may provide an implicit characterization ofthe I-V characteristics of the PV cells. Moreover, the output signaturesmay also include implicit characterizations of the integral powerconverter including initial charging of energy storage devices, startupwattage criterion, algorithmic latencies, etc. Thus, the inventiveprinciples may enable the testing of the operation of all of thecomponents of a PV module even though the output terminals of the PVcells may not be accessible. That is, opto-impulse electronic timesignatures may be used in which, for a typical optical impulse from anarc lamp to a PV module with integrated electronics, all signals at theobservable nodes are sampled using a high-speed data acquisition system.The signatures from multiple periodic and non-periodic flash exposuresas described above may be referred to as multi-opto-impulse extendedsignatures.

In the embodiments described above and below, the output signatureswould typically include voltage and/or current measurements taken fromthe output terminals of a power optimizer, microinverter, or any otherform the integral power converter may take. However, the outputsignatures may include any other parameter that provides an indicationof the compliance of the module under test with manufacturingtolerances. The output signatures may be obtained with any suitable loadapplied to the power converter running the entire range from opencircuit to short circuit. Moreover, the load or loads may be changed toany suitable levels at different times during the test procedures. Forexample, in the embodiments of FIGS. 5-7, a different load may beapplied to the integral power converter before, during or after eachsuccessive light pulse. In general, the load can be active or passiveand may able to act as a source or sink. Furthermore, the active orpassive load system may be synchronized with the flash generationsystem.

FIG. 8 illustrates another embodiment of a system for testing a PVmodule having an integral power converter according to some inventiveprinciples of this patent disclosure. The embodiment of FIG. 8 includesthe same elements as the embodiment of FIG. 2, but in the embodiment ofFIG. 8, a second reference output signature 30 for the module design 10is obtained in response to a second flash pattern 32. During testing,the second flash pattern 32 is applied to the PV module under test 20,and a second observed output signature 34 of the PV module under test 20is acquired in response to the second flash pattern 32.

The second observed output signature 34 is compared to the secondreference output signature 30 through comparison logic 36. As in theembodiment of FIG. 2, the first observed output signature 26 is comparedto the first reference output signature 16 through comparison logic 28.The outputs from comparison logic 28 and 36 may then be furtherprocessed through additional comparison and/or decision making logic 38to determine whether the module under test falls within acceptabletolerances of the target specifications.

As with other embodiments, the module design 10 may be a physical orsimulated embodiment, the reference output signatures may be obtainedthrough simulation or observation of a known good module, any of thelogic may be implemented with hardware, software, firmware, or anysuitable combination thereof, etc.

FIG. 9 illustrates another embodiment of a system for testing a PVmodule having an integral power converter according to some inventiveprinciples of this patent disclosure. The embodiment of FIG. 9 issimilar to that of FIG. 8, but rather than comparing the individualobserved output signatures to their respective reference outputsignatures, the first and second reference output signatures arecombined by logic 41 to generate a composite reference output signature40, and the first and second observed output signatures are combined bylogic 43 to generate a composite observed output signature 42. Thecomposite observed output signature 42 is then compared to the compositereference output signature 40 with comparison logic 44 to determinewhether the module under test falls within acceptable tolerances.

The individual output signatures may be combined to generate compositeoutput signatures using time domain techniques, frequency domaintechniques, time-frequency domain techniques or any other suitabletechnique.

FIG. 10 illustrates an example embodiment of a test method in whichflash patterns A and B are applied to a module design and/or moduleunder test to generate extended output signatures A and B which areshown as generic waveforms for purposes of illustration. The extendedoutput signatures A and B may be combined in the time domain as shown inFIG. 10. The combined extended output signatures may be adequate tocharacterize the response of the module under test, especially withmodules that exhibit high levels of linearity.

Alternatively, the extended output signatures A and B may be combined inthe frequency domain as shown in FIG. 11, where each of the extendedoutput signatures are transformed to the frequency domain, shown here asgeneric spectral profiles. The spectral profiles may then be added togenerate a combined spectral profile. The frequency domain approach mayenable the verification of operating characteristics of modules undertest that may be difficult or impossible to recognize using time domaincombination, especially with modules that exhibit high levels ofnonlinearity.

The flash patterns may also be combined in any suitable manner. Forexample, FIG. 12 illustrates an embodiment in which first and secondflash patterns 18 and 32 are combined by logic 47 through addition inthe time domain to generate a composite flash pattern 46 which may thenbe applied to a module design or module under test, and the resultingsignature analyzed in the time and/or frequency domain or other suitablemethod. FIG. 13 illustrates an example embodiment in which first flashpattern A and second flash pattern B are combined in the time domain togenerate a composite flash pattern A+B.

The flash patterns may be physically combined in any suitable manner.For example, a first pattern A may be generated with a first flash lamp,while a second pattern B may be generated simultaneously with a secondflash lamp. Thus, there may be different time periods when the moduleunder test is illuminated only by the first lamp, only by the secondlamp, or by both lamps at the same time. Alternatively, a single flashlamp may be used to generate the composite pattern by operating at afirst radiance level when only pattern A is active, a second radiancelevel when only pattern B is active, and a third level when patterns Aand B are both active.

Although the embodiments of FIGS. 8-13 are illustrated with only twodifferent flash patterns, any suitable number of flash patterns may beused in accordance with the inventive principles of this patentdisclosure. Moreover, one or more sub-sets of techniques, signaturesand/or flash patterns may be locally determined to optimize testcoverage with a minimum of equipment and test time during manufacture ofmodules having integral power converters according to the inventiveprinciples of this patent disclosure.

As a further enhancement, signatures may be obtained and acquired byshadowing portions of the arrangements of PV cells using shadow patternshaving various shapes and transparencies. For example, first referenceand observed signatures may be obtained and acquired in response to afirst flash pattern with no shadowing of the module design or moduleunder test. Second reference and observed signatures may then beobtained and acquired in response to a second flash pattern with apredetermined shadowing pattern applied to the module design and moduleunder test. The first and second flash patterns may be the same ordifferent. The observed signatures may then be compared to thecorresponding reference signatures as, for example, in the embodiment ofFIG. 8 described above. Alternatively, the signatures may be combined togenerate composite signatures prior to comparison, for example, as shownin the embodiment of FIG. 9.

FIGS. 14A through 14C illustrate another method and apparatus fortesting a PV module having an integrated power converter according tosome inventive principles of this patent disclosure. Referring to FIG.14A, a PV module under test 50 includes an arrangement of PV cells 52and an integrated power converter 54 to process power from the cells.The power converter 54 includes one or more parameters 56A, 56B that arepreset to predetermined states prior to application of a flash pattern.The parameters may include node voltages, bias currents, logic states,power supply states, and any other parameters that, by presetting themto predetermined states, may facilitate the generation of an outputsignature that is useful for evaluating a module under test in responseto one or more flash patterns.

Referring to FIG. 14B, a flash pattern 58 is applied to the module undertest 50, and an observed output signature 60 of the PV module under testis acquired in response to the flash pattern. The presetting of theparameters to predetermined states is now shown in broken lines toindicate that the parameters may be released prior to applying the flashpattern, or may be maintained in the predetermined states during theapplication of all or part of the flash pattern.

The method and apparatus described so far in the context of FIGS. 14Aand 14B relate to a module under test, but the same or similar methodand apparatus may be used to obtain a reference output signature for acorresponding module design, either by simulation or by applying theflash pattern to an actual known good module.

Referring to FIG. 14C, the observed output signature 60 of the PV moduleunder test is compared to the reference output signature 62 throughsuitable comparison logic 64 to determine whether the module under testfalls within acceptable tolerances of the target specifications.

The flash pattern 58 may include multiple light pulses, in which casethe reference output signature and the observed output signature mayinclude extended output signatures. The one or more parameters 56A, 56Bmay be preset a first time before a first one of the light pulses andmay be preset a second time before a second one of the light pulses. Forexample, as shown in FIG. 15, different preset patterns A, B, C and Dmay be applied to the parameters before each of the multiple lightpulses which generate the output signature shown in FIG. 15.

FIG. 16 illustrates an example embodiment of a PV module having anintegrated power converter 66 according to some inventive principles ofthis patent disclosure. The power converter 66 includes a power train 68having a power switch Q1 and an inductor L1 arranged in a buckconfiguration with a diode D1 and output capacitor C1. The power train68 is arranged to convert power from the PV cells 52 to a differentvoltage and current at power output terminals 70 and 72, which arebrought out to high-current terminals on the housing of the powerconverter 66.

The power converter also includes a gate drive circuit 74 to drive theswitch Q1 in response to a controller 76. A house keeping power supply78 taps a small amount of power from the PV cells 52 to provideoperating power to the controller 76 and gate drive circuit 74.

A connector 80 provides access to certain nodes through the housing ofthe power controller. In this example, access is provided to a node COMat a common connection node, a node GD on capacitor C2 at the powersupply input to the gate drive circuit 74, a node CTRL on capacitor C3at the power supply input to the controller 76, and a node HK at aninput to the house keeping power supply 78.

In one example implementation, capacitors C2 and C3 are precharged tonormal operating levels through GD and CTRL while the house keepingpower supply is bootstrapped through HK. Nodes GD and CTRL are thenreleased just before the flash pattern is applied. This enables thepower converter to operate normally without any latency when the flashis applied to the PV cells 52. In another example, the GD connection maybe arranged to cause the gate drive to switch Q1 to always remain onthereby enabling the power train during the flash pattern to simulate,to the extent possible, the raw output from the PV cells 52 appearing atthe power output terminals 70 and 72 through the switch Q1 and inductorL1.

In general, if the controller is implemented digitally, e.g., using amicrocontroller, an additional digital interface may be included topreset digital states of the controller, thereby enabling easier testingof the entire PV Module.

The inventive principles of this patent disclosure have been describedabove with reference to some specific example embodiments, but theseembodiments can be modified in arrangement and detail without departingfrom the inventive concepts. Such changes and modifications areconsidered to fall within the scope of the following claims.

What is claimed is:
 1. A photovoltaic (PV) module, comprising: at leastone PV cell; an integrated power converter configured to process powerfrom said at least one PV cell and provide power at an output thereof;and an accessible node configured to enable a parameter of saidintegrated power converter to be preset to a predetermined state priorto application of a test flash pattern to said PV module.
 2. The PVmodule as recited in claim 1 wherein said accessible node is separatefrom said output.
 3. The PV module as recited in claim 1 wherein saidparameter comprises at least one of a node voltage, a bias current, alogic state and a power converter state.
 4. The PV module as recited inclaim 1 wherein said parameter enables a housekeeping power supply insaid integrated power converter.
 5. The PV module as recited in claim 1wherein said integrated power converter comprises a drive trainconfigured to be enabled through said accessible node.
 6. The PV moduleas recited in claim 1 wherein said accessible node is positioned on aconnector of said integrated power converter.
 7. The PV module asrecited in claim 1 wherein said parameter is operable to be releasedprior to application of said test flash pattern.
 8. The PV module asrecited in claim 1 wherein said parameter is operable to be maintainedin said predetermined state during application of said test flashpattern.
 9. The PV module as recited in claim 1 further comprising aplurality of test flash patterns.
 10. The PV module as recited in claim9 wherein said parameter is preset to different predetermined statesprior to application of said plurality of test flash patterns.
 11. Amethod of operating a photovoltaic (PV) module, comprising: providing atleast one PV cell; processing power from said at least one PV cell withan integrated power converter and providing power at an output thereof;and enabling a parameter of said integrated power converter via anaccessible node to be preset to a predetermined state prior toapplication of a test flash pattern to said PV module.
 12. The method asrecited in claim 11 wherein said accessible node is separate from saidoutput.
 13. The method as recited in claim 11 wherein said parametercomprises at least one of a node voltage, a bias current, a logic stateand a power converter state.
 14. The method as recited in claim 11further comprising enabling a housekeeping power supply in saidintegrated power converter.
 15. The method as recited in claim 11further comprising enabling a drive train of said integrated powerconverter.
 16. The method as recited in claim 11 further comprisingpositioning said accessible node on a connector of said integrated powerconverter.
 17. The method as recited in claim 11 further comprisingreleasing said parameter prior to application of said test flashpattern.
 18. The method as recited in claim 11 further comprisingmaintaining said parameter in said predetermined state duringapplication of said test flash pattern.
 19. The method as recited inclaim 11 further comprising a plurality of test flash patterns.
 20. Themethod as recited in claim 19 further comprising enabling said parameterto be preset to different predetermined states prior to application ofsaid plurality of test flash patterns.